A common problem after performance of a percutaneous transluminal coronary angioplasty is the restenosis of the treated area. In fact, restenosis occurs in 30% to 50% of cases. Restenosis occurs, at least in part, as a result of vascular smooth muscle cell migration, proliferation, and neointima formation at the site of the angioplasty. It has been shown that intracoronary delivery of ionizing radiation causes focal medial fibrosis, which when delivered at the site of the angioplasty, impedes the restenosis process. Adjacent coronary segments and the surrounding myocardium are undamaged by the irradiation treatment.
Delivery of the ionizing radiation at the site of the stenosis can be achieved by the introduction of an irradiation ribbon through an infusion catheter. In known systems, the infusion catheter is inserted to the site of the stenosis over a guidewire which may be inserted before, or left after, the performance of an angioplasty procedure. After insertion of the infusion catheter, the guidewire is removed, and the irradiation ribbon is inserted in its place. The irradiation ribbon typically incorporates a plurality of Iridium-192 seeds or pellets near its distal end. Other sources of ionizing radiation can be used, as well. This plurality of radioactive sources arranged essentially in a line approximates a line source, although the intensity of the radiation will vary axially to some extent, depending upon the spacing and length of the seeds. The irradiation ribbon is inserted to the point where the radioactive material is placed in the area of the stenosis. The Iridium-192 emits gamma radiation having a range of energies between 296 and 612 thousand electron volts (keV).
The currently known systems have several disadvantages. First, the guidewire must be withdrawn before insertion of the irradiation ribbon. Withdrawal of the guidewire is not favored by physicians because it adds at least one step to the procedure, and because it takes additional time. The performance of any additional step presents additional opportunities for complications. Time is of the essence during angioplasty because much of the procedure involves at least partial blockage of the flow of blood in the blood vessel, which can be harmful to the muscle served by the vessel. This problem is compounded during the irradiation procedure, since the radioactive source must often be left in place for several minutes in order to deliver the desired dose of radiation to the vascular tissue. The time problem can be further compounded by the need to reinsert the guidewire after delivering the radiation, in some cases.
A second disadvantage of known systems is that the irradiation ribbon is exposed to blood flow in the infusion catheter, and it is even possible that some of the radioactive seeds could be lost out the distal end of the infusion catheter, or the irradiation ribbon itself can break.
A third disadvantage is that location of the radioactive material radially within the blood vessel is largely uncontrolled. Rotation of the infusion catheter may assist in centering the radiation source within the stenosis, in some cases, but this method is not always effective. Centering of the radioactive material within the tissues injured by the angioplasty may be required, because it is important to deliver a known dose of radiation uniformly to the affected tissue. The intensity of gamma radiation emanating from a line source varies inversely with the square of the radial distance from the source. Therefore, if the radiation source is not centered within the blood vessel, the dose delivered to one side of the vessel can vary greatly from the dose delivered to the opposite side. In addition, if the line source lies at an angle to the centerline of the vessel, rather than being concentric therewith, or at least parallel thereto, the dose delivered can vary axially by an appreciable amount, throughout the length of the stenosis. In some cases, it can even be desirable to position the radiation source parallel to, but offset from, the centerline of the blood vessel, if it is desired to irradiate one side of the stenosis more than the other side. This can be desirable if restenosis is expected to result more from proliferation of the tissues on one side than on the far side.
It is an object of the present invention to provide a catheter assembly for irradiation of a stenotic segment of a blood vessel, which can be inserted to the site of the stenosis over a guidewire. It is a further object of the present invention to provide a catheter assembly which can place an irradiation source at a desired location within a blood vessel, both axially and radially. It is a yet further object of the present invention to provide an irradiation catheter assembly which is economical to manufacture and easy to use.